1. Field of the Invention
The present invention relates to an automatic connection method for an optical network unit (ONU) or an optical network termination (ONT) in an asynchronous transfer mode—passive optical network (ATM-PON) system.
2. Description of the Related Art
Introducing an ATM-PON system is progressing nowadays as one method of realizing a high speed broadband of an access network. The ATM-PON system is a network system in which a plurality of user side apparatus (ONU: optical network unit) and one station side apparatus (OLT: optical line termination) are connected by optical fibers including a splitter in a star type and communications are performed in a connection section by an ATM communication scheme. In the ATM-PON system, optical signals transmitted from ONU to OLT are subjected to time division multiplexing (TDM). Namely, each ONU is assigned a cell slot permitting transmission of an optical signal, and each ONU transmits an optical signal of its own apparatus in accordance with the assigned cell slot to prevent collision with optical signals of other ONUs.
ITU-T Recommendations G.983.1 “Broadband optical access systems based on Passive Optical Network (PON) describes a series of processes called a ranging flow for establishing communications between OLT and ONU and installing ONU. The ranging process provides a function of determining the cell slot of each ONU by measuring a distance between OLT and ONU and calculating a transmission time of an optical signal. When the distance is measured in the ranging process, it is necessary to make one ONU after another respond in order to avoid collision of ONU responses during measurements. To this end, OLT utilizes a unique serial number assigned to each ONU. Namely, in the ranging process, OLT stores the unique serial number of ONU in a signal to be transmitted to ONU, and ONU received this signal responds if the serial number is its own serial number. ONU whose serial number is recognized by OLT and whose distance can be measured in the ranging process is assigned an identification number called a PON-ID, and thereafter messages are exchanged between OLT and ONU by identifying ONU by PON-ID.
This unique serial number of ONU is defined by 64-bit information and defines that upper 32 bits indicate a vendor ID (a fixed value for each vendor) representative of a vendor, and the lower 32 bits are a vendor definition field which the vendor can define arbitrarily. For example, a manufacture year, month and day and a serial number are assigned in the 32-bit vendor definition field, and the serial number is assigned in all 32 bits, etc., in such a manner that each ONU has a different serial number to allow ONU to be identified.
As described above, the ranging process is required for communications between OLT and ONU, and OLT is required to recognize the serial number of each ONU to execute the ranging process. As a method of making OLT recognize the serial number of each ONU, two methods (Method A, Method B) are disclosed, for example, in 8.4.1.1 of ITU-T Recommendations G.983.1.
Method A registers the serial number of each ONU from an operation apparatus (OpS) to OLT. A conventional ATM-PON system uses actually this Method A for ONT connection. Although ITU-T Recommendations G.983.1 does not show an actual concrete operation procedure, the operation procedure is, for example, as follows when Method A is used. First, in order to register an ONU serial number from OpS to OLT, an ONU work person on duty confirms an ONU serial number on an ONU label when ONU is installed, and notifies an OpS person on duty of the ONU serial number by a phone or the like. Next, the OpS person on duty received this notice registers the ONU serial number from OpS to OLT to establish connection between OLT and ONU. In order to avoid complicatedness of the Method A operation procedure, JP-A-2004-214928 proposes a method of reading a bar code displaying an ONU serial number and installing an external apparatus for automatically setting the read ONU serial number to OpS, between ONU and OpS.
Method B searches an ONU serial number on the side of OLT. ITU-T Recommendations G.983.1 introduces an ONU serial number search method by which various bit patterns are simply tried among 64-bit patterns of ONU serial numbers and a bit pattern of a connected ONU is found on the basis of whether there is a response from ONU.
As described above, the operation procedure of ONU connection of Method A of ITU-T Recommendations G.983.1 is complicated and manually executed so that there is a possibility of erroneous operation and erroneous registration. Although the method of JP-A-2004-214928 can prevent erroneous operation and erroneous registration, it requires au automatic connection apparatus as a new external apparatus.
As the ONU serial number search method of Method B, ITU-T Recommendations G.983.1 presents a method by which ONU serial number search from OLT is made distinguishable between a state of “ONU output collision” and a state of “no ONU and no ONU response” and an ONU serial number is searched by increasing or decreasing the search target bit of the ONU serial number by using a binary tree mechanism. The state of “ONU output collision” is intended to be a state that a plurality of ONUs respond and responses are superposed on a transmission line, whereas the state of “no ONU and no ONU response” is intended to be a state that ONU having an ONU serial address designated by OLT does not exist under the control of OLT.
However, in practice, if a plurality of ONUs respond to an inquiry from OLT and the state enters the state of “ONU output collision”, a plurality of ONU responses are superposed on the transmission line and even a timing frame cannot be derived so that OLT cannot receive a response signal. In this case, OLT simply recognizes as the state of “no ONU response” erroneously so that both the states cannot be distinguished and ONU connection through ONU search by the binary tree mechanism is impossible as opposed to expectations.
If all ONU serial number patterns are searched simply by Method B, it is necessary to search patterns of 2 raised to power of 64 (≈18×1018 patterns) because the ONU serial number is defined by 64 bits, and this is not realistic. Even if the vendor ID field (upper 32 bits) of an ONU serial bit is fixed and only the vender definition field (lower 32 bits) is searched, it is necessary to search patterns of 2 raised to power of 32 (≈0.42 billion patterns), and this is also not realistic.